Human breast cancer cell proliferation is regulated by the interaction of a variety of steroid and polypeptide hormones. In particular, treatments designed to reduce the estrogen level or to block estrogen's effects on the tumor with antiestrogens frequently cause tumor regression. A better understanding of the mechanisms by which estrogens and antiestrogens regulate breast cancer cell proliferation would have important clinical implications and might provide clues to new treatment approaches. We therefore propose to use human breast cancer cell lines in vitro and cells growing as tumors in athymic nude mice to study mechanisms by which endocrine manipulation inhibits tumor growth and possibly interfers with simultaneous cytotoxic drug treatment, and how breast cancers eventually escape from endocrine suppression. (1) Studies will be performed to determine whether estrogen deprivation or antiestrogen therapy inhibit growth by a cytostatic mechanism, resulting in accumulation of cells in either G or G1 phase, or by a cytocidal mechanism resulting in cell death. (2) Additional studies of the interaction between the antiestrogen tamoxifen and several cytotoxic agents will be done to clarify mechanisms for the antagonism observed in preliminary studies with melphalan and 5-fluorouracil, drugs used frequently in combination with tamoxifen in the clinic. (3) Finally, the acquired endocrine independence which appears after extended endocrine treatment in cultured cells, in tumors growing in nude mice, and in patients with breast cancer, will be investigated. Initial tumor heterogeneity, loss or alteration of receptors, more distal changes in estrogen-regulated pathways, or changes in the host will be considered. The possibility that altered autocrine growth factor activity is responsible for acquired endocrine independence will be examined, in view of recent data suggesting that polypeptide growth factors secreted by breast cancer cells may mediate the effects of estrogen and may account for the autonomy of hormone-independent tumors. These studies could lead to new strategies to better exploit endocrine responsiveness in breast cancer, to avoid interference between components of combined chemoendocrine therapy, and to prevent or counteract the development of resistance to endocrine therapy.